The present invention relates to a method of inhibiting fouling of heat transfer surfaces in a multi-stage evaporation plant of alkaline waste liquor in chemical pulp industry by heat-treating calcium-containing liquor to be fed to the evaporation plant.
Alkaline cooks, in particular the sulphate process, have conventionally been performed in such a way that the wood, i.e., chips, and the cooking liquor are mixed into a cooking vessel, the temperature of which is then raised to the cooking temperature, typically to 160-180.degree. C., for a given time, whereby lignin is detached from the wood, the result being so called chemical pulp and waste liquor. In a sulphate process, the cooking liquor is white liquor, which is a mixture of NaOH and Na2S. The waste liquor is black liquor containing dissolved lignin, salts of organic acids, extractives and a significant amount of inorganic compounds.
As a rule, the inorganic components are recovered from the black liquor, whereas the organic part is used as combustible matter. For recovery, the black liquor is evaporated to a higher dry matter content (70-90%) in a multi-stage evaporation plant. An evaporation plant operates most commonly in such a way that the vapor produced in the evaporator having the highest pressure is used to heat the evaporator having the next highest pressure. Typically, an evaporation plant comprises 5-7 stages.
A common problem in black liquor evaporation plants is the fouling of heat transfer surfaces, which decreases the efficiency of the heat transfer process. Such fouling is fairly typical on the side of the material to be treated, i.e., black liquor, and more rapid in connection with higher dry matter contents. In order to make the evaporation plant operate satisfactorily, the fouled surfaces need to be cleaned from time to time.
The worst problems with fouling in black liquor evaporation plants are connected with calcium carbonate scaling. Scales caused by calcium mainly consist of crystal calcium carbonate scales or binary salt scales of calcium carbonate and sodium carbonate. Calcium carbonate is a very poorly soluble compound and the dispersed particles in the liquor thereof do not cause fouling.
It has been observed that calcium carbonate scaling is more dependent on the temperature than on the dry matter content of the liquor. Frederick and Grace (Southern Pulp and Paper Manufacturer 42(1979)8:22) have suggested that the amount of dissolved calcium in black liquor is increased because calcium forms a complex together with the lignin in the black liquor. The lignin fragments formed during the cook and containing adjacent hydroxyl groups especially in an aromatic cyclic structure react with calcium. This complex is unstable at high temperatures, whereby it decomposes and releases the calcium ion to the vicinity of the hot surface, whereby the calcium ion rapidly associates with the carbonate ion in the liquor, forming a scale on the surface.
Frederick and Grace have further suggested that the calcium scaling could be reduced or inhibited by means of a treatment in which the black liquor is heated to about 150-160.degree. C. and maintained at this temperature for 10-20 minutes.
The above-described method, in which the black liquor is heated to the temperature of about 150.degree. C., has not been widely used, since the additional energy required for it may cause remarkable costs.